I'm going to try and make this concise: I remove the side plate of my father's computer, who reported it to spontaneously turn itself off after minutes of work (or sooner) and see the CPU heatsink lying on the video card. I reinstall the heatsink without reapplying thermal paste, as there still is plenty of it on the heatsink (and none to be found on the CPU, to me it looked like as if the heatsink had never touched the cpu), CPU keeps reporting overheating @ POST.
Does thermal paste go bad after exposure to air? Should I bother reapplying it or is the CPU bricked? Can I use acetone instead of isopropyl alcohol to wipe off the thermal paste?
Extra info: the PC had been used for over a year, and right now its 34 C (93 F) outside.